Cryptographic systems are generally used when the sender wishes to insure the secrecy and authenticity of the messages to be sent. This is particularly true when the message is to be transmitted over a nonsecure channel, such as a radio or microwave communication link, which can be monitored at will by any adverse party.
The two requirements, secrecy and authenticity, are very different. For example, secrecy prevents the adverse party from deciphering the monitored or intercepted message to obtain the secret message, whereas authenticity assures the receiver of the message of the identity of the sender. For example, an airborne strike force needs a secure system to protect the secrecy of communications sent via radio or data link to the aircraft, and the aircraft needs to be able to authenticate the sender of the communication before responding thereto.
The two most significant developments of the past ten years in the field of cryptography have been the development of the data encryption standard (DES) and public key cryptography. The real significance of DES was not so much the algorithm itself, but the fact that a "standard" was established. DES is able to accommodate high data rates, in excess of ten million bps, but is dependent upon the distribution of an encryption key, which renders it susceptible to potentially fatal key distribution problems. That this is particularly true of covert operations wherein widely disbursed units, some of which may be in hostile territory, must be communicated with.
Public key cryptography has done much to obviate the key distribution problem. The two approaches that have received most attention are the RSA and Knapsack algorithms. A third approach proposed in the literature is the use of the linear Goppa code algorithm. The Knapsack algorithm, which was never commercially implemented, has recently been broken. The most popular system, the RSA algorithm, while it seems to be a strong shield for the data it protects, is extremely slow. The linear Goppa code algorithm possesses a high data rate, in excess of one million bps, and has been strongly indicated to be computationally secure.
The known public key encryption algorithms provide computationally secure encoding algorithms that can be read only by one who has generated and published the public key, or who has access to the decoding algorithm. However, public key systems, by their nature, do not provide simultaneous authentication.
A tutorial introduction to contemporary cryptography may be found in an article Privacy and Authentication: An Introduction to Cryptography by Diffie and Hellman, proceedings to the IEEE, Vol. 67, No. 3, Mar. 1979, Page 397-427.
U.S. Pat. No. 4,351,982 entitled RSA PUBLIC KEY DATA ENCRYPTION SYSTEM HAVING LARGE RANDOM PRIME NUMBER GENERATING MICROPROCESSOR OR THE LIKE, is representative of a number of patents which disclose and teach the use of an RSA Public Key algorithm for cryptographic systems.;
U.S. Pat. No. 4,218,582 entitled PUBLIC KEY CRYPTOGRAPHIC APPARATUS METHOD, is representative of a number of references which teach the use of the Knapsack algorithm in a public key cryptographic system.
The use of linear Goppa codes for a public key algorithm is taught in an article entitled "A Public-Key Crypto System Based on Algebraic Coding Theory", published by R.J. McEliece, DSN Progress Report 42-44, Jet Propulsion Lab, California Institute of Technology, Pasadena, California, January-February, 1978.
U.S. Pat. No. 4,417,338 entitled CRYPTOGRAPHIC KEY SHARING CIRCUIT AND METHOD USING CODE CORRECTION, teaches the use of encoding the encryption key into a code vector which is corrupted by combining it with an error vector to yield a 0 corrupted code vector. The corrupted code vector is corrected by a combination of an error correcting code and the use of at least K of the secondary keys.
Historically, the error vector was viewed as an impediment to the communication process. A "sender" would generate a binary message vector v and send it through a communications channel. What the "receiver" got at the other end was often not the original message v, but rather v+e, that is, the message corrupted by channel induced errors. The goal of error correcting codes was to discover the error vector e and remove its affect from the received vector r=v+e to obtain the original message v.
The book Error Correcting Codes by Petersen & Weldon, Jr., published by MIT Press, Cambridge, Massachusetts in 1972, provides an excellent summary of error correcting codes, particularly those intended for a digital communications environment.